藏北阿里地区改则县雀康一带纳丁措组火山岩锆石U-Pb年龄和地球化学特征

Nadingco Formation volcano rocks zircon U-Pb age and geochemical characteristics in Gêrzê county area of northern Tibet area

纳丁措组火山岩分布于班公湖—怒江结合带以北,属于羌塘—三江复合板片内多玛地层分区雀康地层小区的火山岩地层,代表了羌南地区古近纪的火山活动,为一套粗安岩-粗面岩的岩石组合。在地层区采集了3件岩石年龄样品,测年结果分别为(35.99±0.29)Ma、(37.2±0.65)Ma、(35.23±0.37)Ma。样品中SiO_2含量均较高,介于58.7%~65.51%之间;铝饱和指数A/NFC为0.046~1.029,普遍较低,显示偏铝岩石的特性;MgO含量普遍略高,介于1.10%~2.95%之间。稀土元素(rare earth element,REE)特征显示,样品中REE总量为243.38×10^(-6)~313.48×10^(-6),说明岩石属于稀土元素偏高型,其中∑[w(Ce)]/∑[w(Y)]为15.24~20.61,属于轻稀土富集型;[w(La)/w(Yb)]_N为26.34~52.01,说明轻、重稀土分异明显;δEu为0.77~0.91,属于弱负异常。Rb、Th等大离子亲石元素富集度高,而Hf和重稀土元素等高场强元素丰度值较低,绝大部分样品分析结果显示Ti、Ta、Nb亏损明显,w(La)/w(Sm)值较为稳定,w(Sm)/w(Yb)为4.3~6.7,比值较低。根据岩石地球化学特征,判断纳丁措组火山岩原始岩浆来源于青藏高原加厚陆壳之下与岩石圈地幔之上的特殊壳幔过渡带,形成环境为造山带附近的大陆拉张的板内初始裂谷环境。

Volcanic rocks of the Nadingco Formation is mainly distributed in the north of the Bangong lake- Nujiang suture zone. It is belong to the volcano rock formation of the Qiangtang- Sanjiang composite plate Duoma stratigraphic division Quegang district formation,and represents the volcanic activity of the south of Qiangtang area in Paleogene。 Its main rock type is latite- trachyte. There are 3 dating samples which show the ages of( 35. 99 ±0. 29) Ma、( 37. 2 ± 0. 65) Ma and( 35. 23 ± 0. 37) Ma. The geochemistry samples exhibit w( SiO_2) = 58. 7%-65. 51 % which is high,A / NFC = 0. 046- 1. 029 which is low,w( MgO) = 1. 10 %- 2. 95 % which is high,∑REE = 243. 38 × 10^(-6)- 313. 48 × 10^(-6),and δ Eu = 0. 77- 0. 91. It indicates that the basalt is slightly enriched in light rare earth elements( LREE) and without remarkable Eu depletion. Large ion affinity rock elements such as Rb and Th are high content. High field- strength elements such as Hf and heavy rare earth elements( HREE)are low content. Most samples shows that Ti,Ta and Nb are low content. w( La) / w( Sm) shows stable content. w(Sm) /w( Yb) = 4. 3- 6. 7,which is low. According to the features of rock geochemistry,the primitive magma originates from the special crust mantle transitional zone,which is based on the thickened continental crust of the Qinghai Tibet Plateau. Its forming is the initial intraplate continental extensional rift environment near the orogenic belt.